CPP head with parasitic shunting reduction

ABSTRACT

The series resistance of a CPP GMR stack can be reduced by shaping it into a small upper, on a somewhat larger, lower part. Because of the sub-micron dimensions involved, good alignment between these is normally difficult to achieve. The present invention discloses a self-alignment process based on first laying down a mask that will determine the shape of the top part. Ion beam etching is then initiated, the ion beam being initially applied from one side only at an angle to the surface normal. During etching, all material on the near side of the mask gets etched but, on the far side, only material that is outside the mask&#39;s shadow gets removed so, depending on the beam&#39;s angle, the size of the lower part is controlled and the upper part is precisely centrally aligned above it.

This is a divisional application of U.S. patent application Ser. No.10/734,422, filed on Dec. 12, 2003, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

FIELD OF THE INVENTION

The invention relates to the general field of CPP magnetic read headswith particular reference to reducing series resistance thereof.

BACKGROUND OF THE INVENTION

The principle governing the operation of most magnetic read heads is thechange of resistivity of certain materials in the presence of a magneticfield (magneto-resistance or MR). Magneto-resistance can besignificantly increased by means of a structure known as a spin valvewhere the resistance increase (known as Giant Magneto-Resistance or GMR)derives from the fact that electrons in a magnetized solid are subjectto significantly less scattering by the lattice when their ownmagnetization vectors (due to spin) are parallel (as opposed toanti-parallel) to the direction of magnetization of their environment.

The key elements of a spin valve are illustrated in FIG. 1. On lowerlead layer is seed layer 11 on which is antiferromagnetic layer 12 whosepurpose is to act as a pinning agent for magnetically pinned layer 13.Next is a copper spacer layer 16 on which is low coercivity (free)ferromagnetic layer 17. Capping layer 18 lies atop free layer 17. Whenfree layer 17 is exposed to an external magnetic field, the direction ofits magnetization is free to rotate according to the direction of theexternal field. After the external field is removed, the magnetizationof the free layer remains fixed.

If the direction of the pinned field is parallel to the free layer,electrons passing between the free and pinned layers suffer lessscattering. Thus, the resistance in this state is lower. If, however,the magnetization of the pinned layer is anti-parallel to that of thefree layer, electrons moving from one layer into the other will suffermore scattering so the resistance of the structure will increase. Thechange in resistance of a spin valve is typically 8-20%.

Earlier GMR devices were designed so as to measure the resistance of thefree layer for current flowing parallel to its two surfaces. However, asthe quest for ever greater densities has progressed, devices thatmeasure current flowing perpendicular to the plane (CPP) have begun toreplace them. For CIP devices, the signal strength is diluted byparallel currents flowing through other layers whereas in a CPP device,the total transverse (series) resistance of all layers, other than thefree layer, should be as low as possible.

It is known that AFM layer 12 together with high resistance seed layer11 contribute most of the series resistance in a CPP-GMR structure.Although its functional unit (free/spacer/pinned layers) has a muchhigher GMR ratio, the entire CPP-GMR structure will have a low GMR ratioresulting from the large resistance of AFM/seed layer. Furthermore, theAFM/seed layers form hot spots that further limit the applied currentdensity that can be employed.

In a related application (application Ser. No. 10/718,373 filed Nov. 20,2003), the CPP structure illustrated in FIG. 2 was disclosed. As can beseen, the free and cap layers have been patterned to have a lower widththan the remainder of the stack so the resulting resistance of such astructure is greatly reduced. A typical spacer material such as Cu hasan electron spin diffusion length about 1500 Angstroms. Within thislength, the spin directions of path-altered electrons remain unchangedso that ΔR can be maintained. The GMR ratio of the structure seen inFIG. 2 is increased due to the reduced series resistance and hot spotsare also be eliminated, thereby allowing higher applied current densityand increased signal.

To simplify the description, all the layers above the spacer layer arecalled the top CPP stack and the remaining layers are called the bottomCPP stack. Using conventional process techniques, this structure hasbeen fabricated using two separate lithography/etching/lift-offsequences. The first step patterns the larger CPP bottom stack while thesecond step patterns the top CPP stack.

There are, however, several problems associated with this approach. Tomaximize the GMR ratio in the CPP structure shown in FIG. 2, both topand bottom CPP stacks must have sub-micron dimensions and they have tobe precisely positioned. As the areal density increases, properalignment at these continuously shrinking dimensions becomes verydifficult. Furthermore, after the first lithography/etching/lift-offsequence, the etched CPP structure is exposed to the environment and issubjected to attack from moisture or chemicals.

The present invention discloses a novel method which will eliminate theabove problems. This technique will also improve edge profiles whileachieving small-dimension alignment so that the desired spin diffusionlength can be obtained on both sides of a CPP GMR sensor.

A routine search of the prior art was performed with the followingreferences of interest being found:

Pang et al., in U.S. Pat. No. 6,496,334, describe using IBE in etchingthe CPP stack. In U.S. Pat. No. 6,294,101, Silverbrook discloses IBErotation during etching. Lederman et al (in U.S. Pat. No. 5,627,704) andDykes et al. (in U.S. Pat. No. 5,668,688) are of interest as having todo with CPP fabrication, but do not mention the IBE etching of thepresent invention.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to provide a CPP GMR read head that has reduced seriesresistance.

Another object of at least one embodiment of the present invention hasbeen to provide a process for manufacturing said read head.

Still another object of at least one embodiment of the present inventionhas been that said process be executed during a single pumpdown.

These objects have been achieved by dividing the GMR stack into upperand lower parts. The upper part is a pedestal that comprises the cap andfree layers while the lower part, starting with the spacer layer, is alarger pedestal that contains all remaining layers.

Alignment between the two pedestals is exact because a self-aligningprocess is used. Additionally, the upper pedestal has sidewalls that aresteeper than those of the lower pedestal. Self alignment is achieved byfirst laying down a mask that will determine the shape of the top part.Ion beam etching is then initiated, the ion beam being applied from oneside only at an angle to the surface normal, with the wafer beingrotated in its plane through an angle of up to 180°. After one or moresuch etch steps, the wafer is rotated 180° and the process is repeated.During etching, all material on the near side of the mask gets etchedbut, on the far side, only material that is outside the mask's shadowgets removed so, depending on the beam's angle, the size of the lowerpedestal is controlled with the upper pedestal precisely aligned to becentrally located.

Because of the very small dimensions involved (upper pedestal has amaximum diameter of about 0.3 microns) precise alignment is verydifficult to achieve in any other way.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical CPP GMR stack of the prior art.

FIG. 2 shows how series resistance in the structure of FIG. 1 can bereduced.

FIG. 3 shows the starting point for the process of the presentinvention.

FIG. 4 illustrates how ion beam etching in the presence of a mask isperformed from two different angles.

FIG. 5 illustrates the etch profile that results from ion beam etchingaccording to the process of the present invention.

FIGS. 6 and 7 show the application of a top lead to the structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the invention begins with the formation of mask 31 on thesurface of capping layer 18 as seen in FIG. 3. The invention thenemploys ion beam etching (IBE) applied at two different angles, 41 and42, (180 degree apart) to the wafer as shown in FIG. 4. During the firstcourse of IBE, a wafer coated with a CPP GMR stack is etched from oneside of a mask (typically, but not necessarily, photoresist). The areathat is blocked by the resist shadow remains intact and only the areaoutside the resist shadow will be etched. To achieve uniform etching,the wafer is also rotated within a predefined arc during the IBE process(about an axis normal to its surface). This predefined arc can rangebetween 3 and 180 degree: In the second IBE step, the orientation of thewafer is rotated 180 degrees and the preceding steps are repeated.

The mask shadowing effect controls the creation of the CPP bottom stackwhile the shape of the CPP top stack is determined by the mask pattern.This alternating cycle can be repeated several times to maintain theetching uniformity. The resulting etch profile is shown in FIG. 5 andwill be discussed more fully below. The total etching time is controlledso that etching of the terrace adjacent to the resist is stopped at thesurface of the spacer layer. The mask can be either a single layer or amulti-layer, the latter being preferred because it is more effective foruse in a liftoff process (see below).

The thickness range of the mask is from 0.1 μm and up so long as saidthickness does not affect the capability of achieving a desired criticaldimension. However, the minimum image layer resist thickness must belarge enough to sustain the ion bombardment so that the criticaldimension can be maintained during IBE process. If necessary, the maskcould be a hard mask made of a material such as alumina that is known tohave a low sputtering yield (along with an underlayer to support liftofflater).

Since the process of the invention utilizes the resist shadowing effectto create the top and bottom CPP stacks, it is self-aligning (betweentop and bottom CPP stacks) and requires only one lithography step.Terraces on both sides of the resist can have an equal length that iscontrolled by the beam angle and the thickness of the photoresist. InFIG. 4, the terrace length (L) that is adjacent to the resist pattern isdetermined by incident beam angle (α) and resist height (h) given inEquation (1)L=h×tan αα  (1)

Because the wafer is side-etched twice, with a 180° rotation in between,the beam angles for both sides are maintained at the same value withrespect to the wafer surface normal.

Continuous wafer rotation is commonly used during an IBE process inorder to achieve good etching uniformity. However, during wafer rotationthe resist shadowing effect results in a tapered edge profile on theetched layer. This tapered edge profile is undesirable if a bias layer(exchange or hard bias) is used to provide horizontal stabilization. Inthis invention, the IBE process is conducted one side at a time so theCPP top stack is exposed to the incident ion beam without any blockagefrom photoresist. As a result, a more vertical edge profile can beachieved.

Both slopes are shaped by alternately rotating the wafer (or ion beam orboth) from −C through 0 to +C degrees. During this relative motionbetween the wafer and the beam, the beam angle (relative to the wafernormal) is varied from −A, through B (a fixed angular value) to +Aaccording to the following relationship:A=B+tan⁻¹[(tan B)/cos C]  (2)

Typically, A has a maximum value of up to about 90 degrees and fixedangle B has a value between about 5 and 45 degrees while C can rangefrom a low of about 3 degrees to a maximum of 180 degrees.

The difference in steepness between the inner and outer slopesoriginates from shadowing effects (no shadowing effect on the innerslope). Both inner and outer slopes can be calculated based on exactlythe same angular oscillation except the shadowing effect needs to beincluded in the calculation of the outer slope. Such a relationshipbetween inner and outer slopes can be used for usage verification of ourmethod.

The disclosed method is to utilize symmetric 180° rotation. Suchrotation results in an equal terrace length for each side of a readersensor, as seen in FIG. 5. Even terrace length on both sides of a readersensor is another signature of our approach.

Thus, the oscillating ion beam of the present invention ensures etchinguniformity. After the IBE process, insulating layer 61 such as alumina,silica, or aluminum nitride is then deposited, followed by the lift-offprocess (FIG. 6). Since this invention employs a self-aligned processthat requires only one lithography step, the etching and depositionprocesses can be done within a single (clustered IBE and IBD) system.This eliminates any risk of exposing the freshly etched CPP stack to themoisture and chemicals from the environment before the secondlithography/etching cycle is completed. Furthermore, it is known thatthe photoresist developer will dissolve oxide formed on the metal layersurface. Any loss or contamination of material on the CPP layers willincrease the complexity of the subsequent process and reduce the etchingprecision.

Typically the pedestals that comprise the top and lower CPP stacks havewidth ratios between about 1 and 12 and the top CPP stack has an aspectratio between about 1.5 and 30 as well as a maximum width of betweenabout 0.05 and 0.3 microns while the lower CPP stack has a maximum widthof between about 0.05 and 0.6 microns. The top CPP stack has a thicknessbetween about 100 and 300 Angstroms and the lower CPP stack a thicknessalso between about 100 and 300 Angstroms. The dimensions listed aboveimply a process that requires precise control of the etching end point.Reduction to a single lithography/etching/lift-off cycle will greatlyincreases the layer precision.

After the lift-off process, thick metal layer 71, to form the upperconducting lead, is overlaid on the patterned CPP GMR, as shown in FIG.7, to complete the device. The resulting read head has a GMR ratio of atleast 1% and a series resistance that is less than about 50 ohms.

1. A CPP magnetic read head, comprising: a seed layer on a lower leadlayer; an antiferromagnetic layer on said seed layer; a pinned layer onsaid antiferromagnetic layer; a spacer layer on said pinned layer; afree layer on said spacer layer; a cap layer on said free layer; saidcap layer and said free layer together forming a first pedestal having afirst maximum width; said seed layer, said antiferromagnetic layer, saidpinned layer, and said spacer layer together forming a second pedestalhaving a second maximum width that is greater than said first maximumwidth; said first pedestal being precisely aligned relative to saidsecond pedestal whereby said second pedestal overlaps said firstpedestal by the same amount everywhere; and said first pedestal havingsidewalls that are steeper than those of said second pedestal.
 2. TheCPP read head described in claim 1 wherein said read head has a GMRratio of at least 1% and a series resistance that is less than about 50ohms.
 3. The CPP read head described in claim 1 wherein said secondmaximum pedestal width is between about 1 and 12 times said firstmaximum pedestal width.
 4. The CPP read head described in claim 1wherein said first pedestal has an aspect ratio between about 1.5 and30.